By far, the easiest way to do this is the 3rd quest in the tutorial campaign, "Nuanced Concepts." You aren't given a choice of deck, but it's surprisingly easy.
Every turn, you can have one of your characters guard by pressing
. The next attack will target a guarding character, which will also deal the damage to the attacking enemy. In that way you can destroy enemies without attacking them.
There are only 2 parts to this quest, making it a very short completion, as opposed to main story quests.
gordiedeviant makes a good suggestion in the comments: "Might be worth noting that to win the second stage, you are required to destroy the 4 'doorway' looking things that show dangers. (cobwebs, overgrown something, treacherous path etc). It's fine to attack these. Just DO NOT attack any enemies unless using the +3 Damage Gandalf card."
Hope this helps!